Conventionally, it is known that a thin film made of a chalcogen material or the like formed on a substrate is locally heated by irradiation with laser light, whereby a phase can be changed between an amorphous phase and a crystal phase having different optical constants (refractive index n, extinction coefficient k), using different irradiation conditions. A so-called phase-change type optical information recording medium using the above phenomenon is being studied and developed actively. In the phase-change type optical information recording medium, an information track of the recording medium is irradiated with a laser beam whose output is modulated between two power levels: a recording level and an erasure level, whereby a new signal can be recorded while a previous signal is being erased. Generally, in such a recording medium, a multi-layer film including layers other than a recording layer is used as an information layer for recording information. For example, a multi-layer film including a protection layer made of a dielectric material or a reflection layer made of metal can be used as the information layer.
The protection layer made of a dielectric material has, for example, the following functions of
(1) protecting a recording layer from mechanical damage from outside;
(2) reducing thermal damage occurring in the case where a signal is rewritten repeatedly to increase the rewritable number of times;
(3) enhancing the change in optical characteristics by using an interference effect due to multi-reflection; and
(4) preventing a chemical change due to the influence of outside air.
As a material for the protection layer achieving the above-mentioned objects, conventionally, an oxide such as Al2O3, TiO2 and SiO2; a nitride such as Si3N4 and AlN; an oxynitride such as Si—O—N; a sulfide such as ZnS; and a carbide such as SiC are proposed. Furthermore, as the material for the protection layer, a material such as ZnS—SiO2 that is a mixture of ZnS and SiO2 also is proposed. Among these materials, ZnS—SiO2 has a considerably small thermal conductivity among the dielectrics, and can minimize the heat diffusion occurring during recording with a laser beam. Therefore, the recording sensitivity is enhanced by using ZnS—SiO2. Furthermore, due to a small internal stress, even when this material is formed into a thick film, cracking is unlikely to occur. This material has high adhesion with respect to a phase-change material layer, so that the film made of such a material is unlikely to peel off even after repeated laser irradiation. For these reasons, ZnS—SiO2 mostly is used as the material for the protection layer.
Furthermore, an interface layer between a recording layer and a dielectric layer is proposed. The interface layer has, for example, the following functions of:
(1) promoting the crystallization of the recording layer to enhance erasure characteristics; and
(2) preventing mutual diffusion between the recording layer and the protection layer (dielectric layer) and enhance durability in repeated recording. The interface layer also needs to have characteristics in which corrosion and peeling from the recording layer are not likely to occur.
As the material for such an interface layer, for example, a nitride of Si or Ge is disclosed (see JP 5(1993)-217211 A and WO 97/34298). These materials are very excellent in the above-mentioned crystal core generation promoting effect and diffusion preventing effect. However, it is reported that due to the insufficient adhesion with respect to the recording layer, the interface layer made of Si—N peels off under high-temperature and high-humidity conditions, and thus, the reliability during long-term use is low (see WO 97/34298). In contrast, an interface layer containing Ge—N as its main component is unlikely to peel off even under high-temperature and high-humidity conditions, and thus, Ge—N is one of the most suitable materials for the interface layer. WO 97/34298 shows that Cr or the like is effective as an additive to Ge—N in terms of moisture resistance. Si also is listed as an example of an additive. However, WO 97/34298 does not disclose the amount of Si added to Ge—N and specific effects obtained by adding Si.
In the above-mentioned recording medium, as basic means for increasing the amount of information that can be accumulated in one medium, there is a method for shortening the wavelength of laser light or increasing the numerical aperture of an objective lens condensing the laser light, thereby decreasing the spot diameter of laser light and increasing the density of a recording surface. Furthermore, in order to increase the recording density in a circumferential direction, mark edge recording is introduced in which the length of a recording mark is information. Furthermore, land and groove recording is introduced in which information is recorded on grooves for guiding laser light and lands between the grooves, so as to increase the recording density in a radial direction. Furthermore, the recording density can be increased even by using a plurality of recording layers. A recording medium including a plurality of recording layers and a recording/reproducing method thereof have already been disclosed (see JP 9(1997)-212917 A, WO 96/31875, JP 2000-36130 A). Furthermore, layer recognizing means and layer switching means are disclosed for recording/reproducing information by selecting one recording layer from a plurality of recording layers (see WO 96/31875).
In a recording medium (multi-layer recording medium) including a plurality of information layers, an information layer closer to a laser light source absorbs light. Therefore, an information layer far from the laser light source records/reproduces information with attenuated laser light. This causes a decrease in sensitivity during recording and a decrease in reflectance and amplitude during reproducing. Thus, in the multi-layer recording medium, in order to obtain sufficient recording/reproducing characteristics with a limited laser power, it is required that the transmittance of the information layer closer to the laser light source is increased, and the reflectance, difference in reflectance (difference in reflectance between a crystal phase and an amorphous phase) and sensitivity of the information layer far from the laser light source are increased.
Recently, a violet laser diode having a wavelength in the vicinity of 400 nm is being put into practical use. Then, an attempt is made to increase the density of a recording surface by applying the laser diode to a light source of a recording apparatus for an optical information recording medium. However, the spot diameter of a laser beam is decreased as the wavelength becomes shorter, and hence, the energy density of the laser beam is increased. Because of this, each layer of the information layers is likely to be thermally damaged during recording. Consequently, in the case of a number of repeated recordings, recording/reproducing characteristics are likely to be degraded. Furthermore, in general, the light absorption of the dielectric material is increased and the transmittance thereof is decreased as the wavelength becomes shorter. Therefore, when the wavelength of a laser beam is short, for example, the transmittance of an information layer in the multi-layer recording medium closer to a laser light source is decreased, and a laser beam with a sufficient power cannot reach the information layer far from the laser light source. Furthermore, since the light absorption is increased in the interface layer, the light absorption in the recording layer is decreased, which results in a decrease in recording sensitivity.
In the case of using an interface layer containing the above-mentioned Ge—N as a main component, characteristics hardly are degraded even when recording is repeated a number of times in recording/reproducing using a laser diode with a red wavelength. Furthermore, the extinction coefficient k of the interface layer at a red wavelength is small (i.e., 0.05 or less), whereby a high transmittance can be ensured. However, the interface layer is likely to be thermally damaged as described above at a violet wavelength. Therefore, the interface layer is degraded due to repeated recording. Furthermore, the extinction coefficient k at a violet wavelength is large (i.e., about 0.2), which makes it difficult to ensure a high transmittance.